(i) Field of the Invention
The present invention relates to an ink follower which is used at the rear end of an ink for an aqueous ballpoint pen directly received in an ink reservoir, and it also relates to a ballpoint pen equipped with the ink follower.
(ii) Description of the Related Art
The viscosity of the ink of an aqueous ballpoint pen is in the range of 0.01 Pa.multidot.sec to 3 Pa.multidot.sec, which is much lower than the viscosity of an oily ballpoint pen having a similar structure, i.e., 3 Pa.multidot.sec to 20 Pa.multidot.sec. Thus, if the pen is allowed to stand upwardly or sideways, the ink leaks out. In addition, the ink of the aqueous ballpoint pen leaks even by a light impact which may soil hands or clothing sometimes, and therefore, in order to prevent leakage, an ink follower is provided. This ink follower is also called "an ink control plug".
In Japanese Patent Application Laid-open Nos. (Sho) 57-153070, 57-200472, 61-57673, 61-145269, 61-151289, 61-200187, 61-268786, 62-50379 and 62-148581, it has been disclosed that gel-like ink control plugs are attached to aqueous ballpoint pens in which an ink is directly received in an ink reservoir.
These ink control plugs can sufficiently prevent the ink from leaking out when the aqueous ballpoint pens are positioned in a vertical or horizontal orientation, and they withstand a light drop impact. Of the light drop impacts, the strongest example is a case where the pen drops on a floor from a desk (about 70 cm). In this case, even a conventional ink follower scarcely gives rise to any trouble, through the ink follower slightly shifts.
However, when the conventional ink follower suffers a strong drop impact, the ink follower will shift, resulting in potential structural failure. For example, a drop of a pen from a shelf at a height of about 150 cm is not so rare, and in this case, the impact is twice or more as much as the drop from the desk (about 70 cm) that is applied to the ballpoint pen. In consequence, the viscosity of the ink follower noticeably lowers, so that this ink follower scatters in the ink reservoir. As a result, the ink may leak. In addition, when the contents in the ink reservoir leak in this way, an excessive pressure is applied to a penpoint or its adjacent parts, so that the ball in the ballpoint springs out from the tip holder at times.
A conventional ink follower is formed by imparting a pseudoplasticity to a non-aqueous base oil by the use of silica fine particles. As this pseudoplasticity is strong, the viscosity behavior of the ink drastically alters by the strong impact, and so the viscosity of the ink is affected by the strong drop impact.
As described above, the light drop impact is not so influential, but a tendency that the mechanism of the ink follower will malfunction due to the strong drop impact is remarkable, as the viscosity of the base oil in the ink follower is low. However, the lower the viscosity of the base oil is, the better the follow-up performance of the follower to the ink is, and conversely, the higher the viscosity of the base oil is, the worse the follow-up performance of the follower to the ink is.
For example, U.S. Pat. No. 4,671,691 discloses a viscoelastic ink follower comprising 49% of polybutene, 49% of a mineral oil and 2% of dimethyldioctadecylammoniumbentonite. Briefly describing this ink follower from the viewpoint of the present invention, a clay-based thickening agent is used to impart the weak pseudoplasticity to the high-viscosity base oil, but the follow-up performance of the follower to the ink is poor, though the impact resistance is excellent.